David Hendley on tue 24 oct 00
Why is firing a claybody far below vitrification undesirable?
In a perfect world, a glaze covers a clay surface and
seals it tight. In fact, it is pretty difficult to achieve
this ideal consistently and completely.
Just go back in the Clayart archives and read all the
questions about sealing oil lamps that seep oil, to
see what a problem it is.
A vitrified claybody makes achieving a perfect glaze
fit less important; even if there are tiny cracks in the
glaze, a vitrified body will not absorb water or seep oil.
Your oven dishes present an even harder glaze fit
problem because repeated heating and cooling in a hot
oven will cause even a 'good' glaze to craze over time.
The glazes studio potters like to use often stretch the
boundaries of good-fitting long-wearing glazes, because
the glazes look pretty. Even a half-cone difference in
firing temperature for some glazes marks the difference
between a good fitting glaze and a crazed glaze.
A vitrified body under these glazes makes the technical
properties of the glaze less (though still) important.
A 'ring' rather than a 'thud' is important for a bowl, as well
as for a wind chime. A water-logged, crazed bowl that sounds
like a watermelon when thumped is pretty disgusting.
A good vitrified claybody will ensure a pleasant tone.
If you are really interested in how your underfired claybody
performs, here is a test:
Soak a well-used mug in water overnight. The next day, boil
a cup of water in the microwave. If the claybody has absorbed
water, the handle will be too hot to hold. In extreme cases,
you will see white or gray stuff coming to the surface, through
cracks in the glaze.
Be careful, and check every 10 seconds, as a mug that has
absorbed moisture will get very, very hot, and, I suppose, could
even explode!
Hopefully your mug will pass the test because either your
glaze fits well and/or the claybody is reasonably close to
vitrification.
--
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com/
----- Original Message -----
| One more example of "primitive" or "unsophisticated" (meaning
| non-western or
| pre-industrial) potters achieving what cannot be done with our current
| technology.
| There seems to be definite disadvantages to firing clay bodies to
| vitrification.
| Sure, it is desireable for wind chimes and may make the ware more impact
| resistant,
| but it makes the ware much less tolerant of thermal shock. Those of us
| who do smoke
| firing and raku know this well. As far as porosity is concerned, isn't
| that what
| glazes are for?
|
| I'd like to hear from the Gurus just what the case is for considering
| firing below
| vitrification undesireable. I regularly use a stoneware clay rated at
| ^8-10 for
| functional ware fired to ^6-7, and find it very practical - use it
| everyday,
| including oven dishes.
|
David Hendley on thu 26 oct 00
Dear Norman van der Sluys ,
Well, by luck or happenstance, your glazes, clay, and
firing temperature work. Good for you.
The difference between 'cone 7 horizontal' and cone 8
is maybe 25 degrees F. Your clay must be adequately
fused at your temperature so everything works out OK.
You asked for the us to tell you the reasons why firing
below vitrification is undesirable. We weren't criticizing
your work, as we haven't even seen it.
All of the faults mentioned, such as bowls that 'thud'
when tapped, mug handles that ooze foam when heated
in the microwave, plates that grow mold, and oil lamps
that seep oil, are real problems traced back to under-
vitrified claybodies.
It's not a matter of 'hotter is better', it's a matter of designing
a claybody that is correct for the temperature of your firing.
A good cone 10 claybody can be made into a good cone 6
claybody with the addition of more feldspar or other fluxes
to increase the vitrification.
Finally, I fire with wood to cone 10-11 because I want to, and I
consider it beneficial and desirable from both a functional and
environmental standpoint. Functional, because a high temperature
is required for pleasing ash and flashing effects, and environmentally
because I am using waste wood for fuel rather than gas or
electricity, which are both polluting and, except in rare instances,
non-renewable.
In actuality my fuel is free and I spend less money to fire than you do.
--
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com/
----- Original Message -----
From: Norman van der Sluys
To:
Sent: Thursday, October 26, 2000 8:01 AM
Subject: Re: firing below vitrification, was "tajine"
| John,
|
| What you say may be true for grossly underfired pieces (of course my
smoke-fired
| things are not functional in the sense of use for holding food or
liquids,) but my
| daily experience contradicts all the caveats this thread has generated
thus far. I
| have never seen evidence of mildew or leaking, even with oil lamps, after
several
| years of everyday use. I use my ware in the microwave extensively and it
seems to
| outlast the glass plates that come with the microwaves! My coffee mug gets
nuked
| several times a day, and food stored in the refrigerator is reheated in
| serving-storage bowls I make. Even a coffee mug with noticeable crazing
passes
| David Hendley's overnight soak then boil test. This despite the fact that
the clay
| I use (Minnesota Clay Northern Light w/iron) is rated as ^8 - 10 and I
typically
| fire to ^6 well down with ^7 horizontal. Yes, my glazes seem to fit the
body well
| at this temperature, and I consider a pot with a glaze miss on the
interior to be a
| second at best, but is this not part of craftsmanship? If a piece is to
be
| considered functional shouldn't the glaze coat be functional as well as
the body
| and the design?
|
| There seems to be a certain high-heat mystique that says "hotter is
better." Most
| of the advocates of this philosophy seem to be heavily influenced by
traditional
| Japanese and Sung Dynasty Chinese ware. From my recent reading (Tichane,
and
| Richard L. Wilson's excellent Inside Japanese Ceramics, among others,) I
gather
| that many of these inspiring pieces were not fired higher than about 1230
oC. I
| have no objection to people firing their stoneware so high that they need
wadding
| or alumina powder to keep unglazed surfaces from sticking to the shelves,
if that
| is the way they want to spend their money, but pracical experience tells
me this is
| neither beneficial nor desireable from a functional or environmental
standpoint.
|
| John Hesselberth wrote:
|
| > Well for one thing it makes it totally unsatisfactory for use in today's
| > microwave world. A pot that absorbs moisture is a dangerous thing in a
| > microwave. Of course you are also totally dependent, then, on the glaze
| > to hold liquids and that means a single craze or glaze defect and you
| > have a leaky pot.
| >
|
| --
| Norman van der Sluys
|
| by the shore of Lake Michigan
|
| |
|
